The Brew Cabin

Hi, have been searching site for possible remedies to my problem, but thought I'd better ask.
I have a detached house , built 1924, with downstairs cloakroom. After several years the gulley where the cloakroom sink discharges has become blocked with sediment so drains away slowly. It is next to the soil pipe and, not knowing much about the drainage system, I lifted the inspection cover located in the driveway to see firstly that there wasn't a blockage, and to see if the water from the sink flowed into the sewer albeit slowly, but could see evidence of neither.
Is it possible that the sink gulley could be draining to a soakaway? The grid covering the gulley is concreted in so that removing the sediment is impossible unless I break up the concrete which I'm loathe to do (want to sell the house this year so any construction work needs to be kept to a minimum!)
I would be grateful for any suggestions and possible remedies.

It's highly unlikely that the cloakroom sink will be drained to a soakaway. Soakaways are only ever used for surface water, NEVER for foul, although mistakes have been made.

Replacing the bunged-up cloakroom gully might cost a couple of hundred quid, but it's not a particularly big job. Your local BM might be able to suggest a contractor that undertkes this type of work.